1. Field of the Invention
The present invention relates to projection optical systems, and in particular to apochromatic large-field unit-magnification projection optical systems for photolithographic applications.
2. Description of the Prior Art
Photolithography is presently employed not only in sub-micron resolution integrated circuit (IC) manufacturing, but also to an increasing degree in advanced wafer-level IC packaging as well as in semiconductor, microelectromechanical systems (MEMS), nanotechnology (i.e., forming nanoscale structures and devices), and other applications. These applications require multiple imaging capabilities ranging from relatively low resolution (i.e., a few microns) with large depth of focus, to relatively high resolution (i.e. sub-micron) and a high throughput.
The present invention, as described in the Detailed Description of the Invention section below, is related to and is an improvement over the projection optical system described in U.S. Pat. No. 4,391,494 (hereinafter, “the '494 patent”) issued on Jul. 5, 1983 to Ronald S. Hershel and assigned to General Signal Corporation, which patent is hereby incorporated by reference.
FIG. 1 is a cross-sectional diagram of an example prior art projection optical system 8 according to the '494 patent. The projection optical system described in the '494 patent and illustrated in FIG. 1 is a unit-magnification, catadioptric, achromatic and anastigmatic, projection optical system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures (NAs). The system is basically symmetrical relative to an aperture stop located at the mirror, thus eliminating odd order aberrations such as coma, distortion and lateral color. All of the spherical surfaces are nearly concentric, with the centers of curvature located close to where the focal plane would be located were the system not folded. Thus, the resultant system is essentially independent of the index of refraction of the air in the lens, making pressure compensation unnecessary.
Optical system 8 includes a concave spherical mirror 10, an aperture stop AS1 located at the mirror, and a composite, achromatic plano-convex doublet lens-prism assembly 12. Mirror 10 and assembly 12 are disposed symmetrically about an optical axis 14. Optical system 8 is essentially symmetrical relative to an aperture stop AS1 located at mirror 10 so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system 8 are nearly concentric.
In optical system 8, doublet-prism assembly 12 includes a meniscus lens 13A, a plano-convex lens 13B and symmetric fold prisms 15A and 15B. In conjunction with mirror 10, assembly 12 corrects the remaining optical aberrations, which include axial color, astigmatism, petzval, and spherical aberration. Symmetric fold prisms 15A and 15B are used to attain sufficient working space for movement of a reticle 16 and a wafer 18.
Optical system 8 also includes an object plane OP1 and an image plane IP1, which are separated via folding prisms 15A and 15B. The cost of this gain in working space is the reduction of available field size to about 25% to 35% of the total potential field. In the past, this reduction in field size has not been critical since it has been possible to obtain both acceptable field size and the degree of resolution required for the state-of-the-art circuits.
In the '494 patent, the doublet-prism assembly corrects the remaining optical aberrations, which include axial color in the g-h band, astigmatism, petzval, and spherical aberration. However, the '494 patent cannot provide a very high quality image for large-field and broad spectral band applications (≧50 mm×100 mm and g, h and I spectral lines), and numerical apertures of 0.15≦NA≧0.20. Moreover, the teaching of the '494 patent also does not provide for a unit-magnification projection optical system with high quality imagery for numerical apertures of 0.2≦NA≦0.40 with a field radius greater than 38 mm for a broad exposure band. The '494 patent also does not provide for achromatization at this broad exposure band and at a visible wavelength, which is desirable for aligning the mask and the wafer in a photolithography system.
The present invention, as described in the Detailed Description of the Invention section below, is also related to and in an improvement over the projection optical system described in U.S. Pat. No. 4,171,871 (hereinafter, “the '871 patent”), issued on Oct. 23, 1979 to Dill et al., and assigned to IBM Corporation, which patent is hereby incorporated by reference.
The projection optical system of the '871 patent is achromatic over a wide spectral band and utilizes a total of five glass types for the lens elements with dioptric powers. The projection optical system of the '871 patent is comprised of three glass types for the first lens group, two glass types for the second lens group, and a mirror. The combination of the second lens group and the mirror constitute what is known in the art of optical design as a “Mangin mirror”. The projection optical system of the '871 patent may be aligned in the green part of the visible spectrum if the exposure system operates at a near UV wavelength without refocusing since the projection system provides two coincident foci over this broad spectral band. This may be contrasted with the present invention, described below, which provides achromatization at two or more discrete wavelengths within the broad ultraviolet (UV) exposure spectral band covering the g, h, and I lines of the mercury spectrum, as well as achromatization simultaneously at another additional discrete visible wavelength where the photoresist is not sensitive.
To address the present-day robust requirements of a photolithography system as discussed above, it is desirable to have a projection optical system capable of providing a large-field, with relatively low-resolution imaging, as well as a system providing a moderate size field, with relatively high-resolution imaging. It is preferable that such a projection optical system provide exposure with diffraction-limited performance over a broad exposure wavelength band covering the g, h, and I spectral lines of mercury (436 nm, 405 nm, 365 nm, respectively) for high-throughput with applications requiring high exposure doses.